Nice attention to detail that you put -san after otouto when Raph spoke it as a form of respect. Leo could also put -chan or -kun when he spoke it. Or Raph could have also said -sama because Leo is ninja clan leader. Anyway, love the lines of Nihongo here.

Wow, you're updating faster than I can read. :) Just for some reason, I got this chapter and Chapter 5 twice.

I noticed one thing that is plausible, but I just don't like the idea, but it's your story. It felt like Donnie jumped too quickly to the conclusion that Mikey stabbed Leo. But it works to keep the storyline going.